Coagulase positive staphylococci are well established agents of infection and the resistance of these organisms to a wide range of antimicrobial agents are well documented.[3] Recent reports indicate that coagulase negative staphylococci may also cause disease and several workers have shown them to exhibit a wide variability in their patterns of resistance.[1], [3], [4], [6], [8] The present study was undertaken to identify and determine the antibiograms of coagulase negative staphylococci isolated from specimens submitted to a large general hospital.

Staphylococci isolated in pure culture, more than once from various sources including wounds, blood, urine, and tissue fluids were included in this study which extended over a period of 13 months. The isolates were determined to belong to the genus staphylococcus on the basis of their being Gram positive, catalase positive cocci, which utilise glucose fermentatively. The isolates were further identified by the scheme recommended by the Subcommittee on the Taxonomy of Staphylococci and Micrococeci.[5] The antibiotic disc susceptibility tests were done by the Kirby Bauer Method and the minimum inhibitory concentration (MIC) for penicillin was determined by agar dilution method.[7] Beta lactamase production for these strains was studied using plate iodometric technique[9] with standard beta lactamase positive and negative controls.

One thousand three hundred and fifty-two strains of staphylococci were isolated from various specimens of which 1300(96.37%) were coagulase positive staphylococci and 52 (3.7%) were coagulase negative staphylococci.Drainage from wounds was the most frequent source of coagulase negative staphylococci (40 strains, 76.92%) followed by blood (6 strains, 11.54%), tissue fluids (5 strains, 9.62%) and urine (one strain, 1.92%).Of the coagulase negative staphylococci, 29 (55.75%) were identified as S. epidermidis and 23 (44.25%) as S. saprophyticus.The results of antibiotic disc susceptibility tests of coagulase negative staphylococci are shown in [Table - 1] these reveal that most strains of coagulase negative staphylococci are sensitive to gentamicin and a fair number to erythromycin though only half of them were susceptible to penicillin.[Table - 2] shows MIC levels and beta lactamase activity of the isolates. Strains reported sensitive to penicillin on antibiotic disc susceptibility test had an MIC level of 0.5 units/ml or less, those reported resistant had MIC levels of 2 units/ml or more. Of the 52 strains, 25 strains were beta lactamase producers and penicillin resistance was associated with beta lactamase activity. One strain identified as S. epidermidis was sensitive to penicillin in the antibiotic disc susceptibility test, had low MIC level (0.25 units/ml) and yet showed the presence of beta lactamase enzyme.

The coagulase negative staphylococci on the basis of the classification used in our study showed fairly equal distribution of S. epidermidis and S. saprorphyticus. Other workers[3], [4] have reported the incidence of S. epidermidis significantly higher than that of S. saprophyticus which may be due to different populations studied.Determination of the susceptibility of the isolates to 7 antimicrobial agents showed that overall, coagulase negative staphylococci tend to be resistant to a wide spectrum of antimicrobial agents. In a recent communication,[10] there is a similar report of resistance of coagulase negative staphylococci, isolated from clinical samples.The fact that beta lactamase activity was demonstrated in one strain reported sensitive on the basis of antibiotic disc susceptibility test is interesting as Thornsberry et al[11] have reported similar strains and have shown them to be clinically resistant to penicillin.The present study suggests that if coagulase negative staphylococci are repeatedly isolated from patients with infection they should be taken seriously and antibiotic susceptibility tests done on these isolates.